Euglena cells grown in diurnal light-dark cycles exhibit circadian variations of their cAMP content, which we believe to be under the control of an endogenous timer because they persist in constant darkness in the absence of any environmental time cue. We think that these cAMP oscillations may play a role in the regulation of some of the numerous cellular activities that are known to display circadian rhythmicities in this organism. The role of cAMP in algal cells is still controversial, however, since the nature of the cAMP "receptor" is unknown. We show that extracts of the achlorophyllous ZC mutant of Euglena gracilis contain two cAMP-binding proteins, which bind cAMP with a high affinity (Kd values of 10 nM and 30 nM) and which can be separated by DEAE-cellulose chromatography. Protein kinase activity was assayed using Kemptide as a substrate. Stimulation of kinase activity by cAMP was observed after partial purification by DEAE-cellulose chromatography. Two peaks of activity were resolved, corresponding to distinct enzymes with different cAMP-analog specificities. Thus, cAMP signaling in plant cells may proceed by the phosphorylation of target proteins by cAMP-dependent kinases, in a manner similar to that of animal cells.